As for a liquid jet recording system (hereinafter occasionally referred to as "ink jet recording system"), there have been proposed a variety of systems. Among such proposals, as a typical one, the public attention has been focused on those liquid jet recording systems disclosed, for example, in U.S. Pat. No. 4,723,129 or 4,740,796 in recent years. These systems are of the type in short that recording liquid (hereinafter occasionally referred to as "ink") is discharged utilizing thermal energy and recording is performed with the liquid discharged. And there are advantages for these liquid jet recording systems that recording of a high quality image with a high density and a high resolution can be performed at a high speed, and it is relatively easy to attain miniaturization of a head or an apparatus.
A typical embodiment of the head in which such liquid jet recording system utilizing thermal energy is applied comprises a discharging outlet for discharging liquid, a liquid pathway in communication with said discharging outlet and having, as part of its constituent, a heat acting portion at which thermal energy, which is utilized for discharging the liquid from the discharging outlet, is effected to the liquid, and an electrothermal converting body which is disposed to correspond to the liquid pathway and which serves to generate the thermal energy to be utilized for discharging the liquid. And this electrothermal converting body has, in general, a pair of electrodes and a heat generating resistor layer disposed to connect said pair of electrodes and to form an exothermic region (heat generating portion). On this electrothermal converting body, an upper protective layer capable of protecting the electrothermal converting body from recording liquid is usually disposed. An example of the constitution of a substrate for liquid jet recording head to be used for the preparation of such liquid jet recording head is shown in FIG. 1(a) through FIG. 1(b).
FIG. 1(a) is a schematic plan view (including partial perspective views for explanatory purpose) of the principal portion of an example of the substrate for liquid jet recording head. FIG. 1(b) is a cross section view taken along the chain line X - Y in FIG. 1(a).
In FIG. 1(a) and FIG. 1(b), a substrate 101 for liquid jet recording head comprises a lower layer 106, a heat generating resistor layer 107, a pair of electrode layers 103 and 104, a first upper protective layer 108, a second upper protective layer 109 and a third upper protective layer 110 being laminated in this order on a support member 105. A portion 111 of the heat generating resistor layer 107 which is situated between the electrodes 103 and 104 forms a heat generating portion. Details of the respective constituents will be described later.
As for the material of which each of the upper protective layers in such substrate 101 for liquid jet recording head is formed, it is properly selected depending upon the characteristics required therefor such as heat resistance, resistance to liquid, thermal conductivity and insulating property. The principal role of the first upper protective layer 108 is to insulate between the electrode 103 and the electrode 104. The principal role of the second upper protective layer is to reinforce the resistance to liquid and the mechanical strength. The principal role of the third upper protective layer is to prevent liquid permeation and to improve the resistance to liquid. And in order to meet such requirements with respect to the characteristics, in many cases, the first upper protective layer is formed of an inorganic insulating material, the second upper protective layer is formed of an inorganic material (specifically, a metallic material), and the third upper protective layer is formed of an organic material.
There have been produced liquid jet recording heads with a relatively high reliability using such substrate for liquid jet recording head as above mentioned, and liquid jet recording apparatus provided with those liquid jet recording heads have been commercialized.
However, for the commercially available liquid jet recording apparatus, there is a societal demand for further improvement in the recording speed and for further improvement in the quality of an image recorded. As one of ideal liquid jet recording heads capable of meeting such social demand, there can be mentioned a liquid jet recording head that is basically provided with numerous liquid discharging outlets as many as possible being arranged at a high density and that can be repeatedly used for a long period of time without deteriorating.
In order to realize such ideal liquid jet recording head, such a matter as will be mentioned in the following as the subject to be solved. That is, in the case of a liquid jet recording head provided with numerous discharging outlets being arranged at a high density, there is a problem that the electrothermal converting bodies including the electrodes are apt to gradually corrode with a recording liquid when it is frequently and continuously used for a long period of time.
The above matter is not serious and can be more or less admitted in the case of a head provided with a relatively small number of discharging outlets being arranged at a relatively low density (for the reason in this regard, it is presumed that the head is substantially small in the number of portions liable to cause a problem and the probability of causing a problem is substantially small). However, the above matter is a subject which cannot be slighted in the case where numerous discharging outlets are arranged at a high density. Particularly, it is a serious technical subject in the case of a so-called full-line type liquid jet recording head which is provided with numerous discharging outlets being arranged at a high density along the entire width of the recording area of a member on which an image is to be recorded in which numerous electrothermal converting bodies are arranged at a high density on a base member such that they correspond to said numerous discharging outlets.